New Wii player, Lego Star Wars question

I'm stuck, and every walkthrough gives me the same thing (which isn't helping).

I'm in Jabba's Palace, fighting the Rancor. I know you use the droids to open the hatches and make the pig-guards come down. The problem is, the Rancor has gone to the lower right corner about 30 times now. Only the first time did he go to the explosive box near the hatch. The box on the right doesn't explode.

What am I doing wrong? Do I have to get it on the first try or he'll keep going right? Is that right box SUPPOSED to explode? I got him under those boulders once, but can't get a second time (after 2 times apparently he'll go under the hatch to be crushed.